This invention relates to an improved waste water drainfield for a sewerage disposal system.
Sewerage disposal systems, such as septic tanks, are usually provided with a drainfield to assist in dispersing waste water. As shown in FIG. 1, and described below, these drainfields typically comprise a plurality of identical absorption trenches that are connected in series. Waste water is disbursed by distributing it to the trenches, where it percolates through the soil. However waste water distribution in a conventional system is uneven. The waste water is typically conducted to the first trench in the series. Since the flow of waste water is generally greater than the percolation capacity of the first trench, the waste water level rises until it spills over to the second trench. Again the flow of waste water, particularly at peak periods, is generally greater than the percolation capacity of the first and second trenches, and the waste water level rises in the second trench as well, and from time to time spills over to the third trench. Thus, in a conventional system, the full percolation capacity of the trenches is not realized until most of the trenches are filled to capacity.
As alluded to above, conventional waste water systems are typically designed for an average waste water flow rate, i.e. the percolation capacity of the trenches in the system is designed to meet the average waste water flow rate. However, as illustrated in FIG. 3, actual water flow can vary significantly from the average flow rate. Unless they are substantially oversized, conventional drainfields have difficulty handling peak waste water flow rates, and there is a risk that the waste water will overflow the trenches, breaking the ground surface, in what is known as "hydraulic failure" or more commonly a "blow out". This condition is exacerbated by rain or flooding, which wets the soil, and thus reduces the water storage capacity of the soil. Moreover, as the trenches fill up, there is a continuous hydrostatic head from the septic tank through the trenches to the last trench. This continuous, unbroken hydrostatic head increases the likelihood that waste water will break the surface of the ground.
Some conventional waste water systems employ a distribution box to divide the waste water flow directly to absorption trenches. However, such distribution boxes are prone to failure. Even if the distribution boxes operate properly, such a system is still subject to overload during peak flow rates, and the trenches are still subjected to a substantial hydraulic head that cause waste water to overflow and break the surface of the ground.
Generally, the waste water drainfield of the present invention includes a plurality of conventional absorption trenches, and a distribution system for distributing waste water to the trenches. This distribution system includes at least one distribution pipe at each of the absorption trenches for temporarily storing a quantity of waste water and for distributing the waste water to its respective absorption trench. There is a distribution device at each of the absorption trenches except one, for separating waste water into first and second flow components of preselected proportions. A plurality of distribution lines connect the distribution devices in series. A distribution line also connects the last distribution device in the series to the distribution pipe for the absorption trench without a distribution device. Each of the distribution devices delivers the first flow component to the distribution pipe for its respective trench for temporary storage and/or distribution. Ea oh of the distribution devices delivers the second flow component via the distribution lines to the next distribution device in the series, except the last distribution device in the series, which delivers the second component to the distribution pipe for the absorption trench without a distribution device.
The distribution pipes provide storage capacity so that the drainfield can a accommodate peak water flow rates without water breaking the surface of the ground. Thus, the drainfield does not have to be oversized to accommodate peak flow rates. Because the trenches are connected in parallel, rather than in series, water distribution through the drainfield is more even. Moreover, the hydrostatic head is interrupted by the distribution devices and thus the hydrostatic head on the trenches is reduced. The distribution devices allow control over the amount of waste water delivered to each trench. This allows the trench size to be varied. This flexibility in trench size makes it easier to design a system for a given site.
These and other features and advantages will be in part apparent, and in part pointed out hereinafter.